1. Field of the Invention
This invention generally relates to the field of optical microscopy imaging which uses selective illumination or excitation. More specifically, this invention relates to a method of using selective excitation patterns optimized for imaging of microparticles.
2. Description of the Related Art
A selective excitation scheme for microparticle imaging requires two laser beams to produce interference patterns (fringes) on the microparticle array plane in each pattern channel and at least two channels at different fringe orientations. In previous designs, fringe patterns at several different orientations on a target plane were implemented by constructing multiple pattern channels on the circumference of a semi-circle or circle (an arrangement referred to as a “ring”). If the combination of the required fringe pitch and the number of channels goes beyond a certain limit, the beam paths between adjacent channels will cross each other. This circumstance of intercrossing beams complicates the system design and challenges the pattern channel alignment. An example of the arrangement of components that results in the intercrossing of beams is illustrated in FIG. 9, wherein the beam paths 901 for channel #1 and 902 for channel #2 cross each other.
To provide a light source to illuminate the pattern channels, conventionally a laser beam is split into several beams simultaneously using a diffractive grating or cascaded splitters and used to illuminate all of the pattern channels simultaneously. However, in real time operation of the apparatus, only one pattern channel is projected onto the microparticle array plane in the acquisition of the microparticle array image. The other pattern channels of the fringes are blocked. As a result, each pattern channel utilizes only a small fraction of the full laser power to illuminate the target samples and the illumination efficiency is significantly reduced.